Systems and methods for coarse and fine time of flight estimates for precise radio frequency localization in the presence of multiple communication paths

ABSTRACT

Systems and methods for determining locations of wireless nodes in a network architecture are disclosed herein. In one example, an asynchronous system includes a first wireless node having a wireless device with one or more processing units and RF circuitry for transmitting and receiving communications in the wireless network architecture including a first RF signal having a first packet. The system also includes a second wireless node having a wireless device with a transmitter and a receiver to enable bi-directional communications with the first wireless node in the wireless network architecture including a second RF signal with a second packet. The one or more processing units of the first wireless node are configured to execute instructions to determine a coarse time of flight estimate of the first and second packets and a fine time estimate of the time of flight using channel information of the first and second wireless nodes.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.15/173,531, filed on Jun. 3, 2016, the entire contents of which arehereby incorporated by reference.

FIELD

Embodiments of the invention pertain to systems and methods for coarseand fine time of flight estimates for precise radio frequencylocalization in the presence of multiple communication paths.

BACKGROUND

In the consumer electronics and computer industries, wireless sensornetworks have been studied for many years. In archetypal wireless sensornetworks, one or more sensors are implemented in conjunction with aradio to enable wireless collection of data from one or more sensornodes deployed within a network. Each sensor node may include one ormore sensors, and will include a radio and a power source for poweringthe operation of the sensor node. Location detection of nodes in indoorwireless networks is useful and important in many applications.

Localization based on triangulation performed using radio frequencymeasurements is an attractive method for determining location ofwirelessly equipped objects in three dimensional space. RF-basedlocalization may be performed in numerous ways. Distances betweenmultiple object pairs must be determined to enable calculation ofrelative positions in three dimensional space via triangulation based onthe individual pair distances. An exemplary implementation includes ahub and multiple sensor nodes. Note that the hub may be replaced with anode, or indeed, one or more of the nodes may be replaced with a hub.Distances are estimated using radio frequency techniques between all theindividual pairs via RF communications. Once the distance is estimated,triangulation may be used to determine the relative position in threedimensional space of each object. If the position of at least 2 of theobjects is known in real space, then the absolute position of eachobject in the network may be determined. Indeed, if the position of 1object (e.g., the hub) is known within the network, along with theangular path to at least one other node, then again the absoluteposition of each object within the network may be determined.

Distance measurement between object pairs is therefore a key step inRF-based localization. Distance estimation may be performed in numerousways. Signal strength of communication (RSSI) may be measured betweenpairs and used to estimate distance based on known models of signalattenuation. Time of Flight (TOF) may be measured for signalstransmitted between objects and distance may be estimated based on knownpropagation delay models. Angle of arrival (AOA) may additionally beestimated based on resolution of angular variation in signal strength.Of these, RSSI is often prone to error due to variations in attenuation,and is therefore less attractive than TOF for distance estimation.

TOF based distance estimation is susceptible to error due to reflectionscausing the presence of multiple paths between two objects. In thissituation, the estimated path may be detected as being longer than thereal path due to the reflected path being longer than the direct path.If the system estimates the TOF based on the reflected path, then errorsare introduced in triangulation.

SUMMARY

For one embodiment of the present invention, systems and methods fordetermining locations of wireless sensor nodes in a network architectureare disclosed herein. In one example, an asynchronous system forlocalization of nodes in a wireless network architecture includes afirst wireless node having a wireless device with one or more processingunits and RF circuitry for transmitting and receiving communications inthe wireless network architecture including a first RF signal having afirst packet. The system also includes a second wireless node having awireless device with a transmitter and a receiver to enablebi-directional communications with the first wireless node in thewireless network architecture including a second RF signal with a secondpacket. The one or more processing units of the first wireless node areconfigured to execute instructions to determine a coarse time of flightestimate of the first and second packets and a fine time estimate of thetime of flight using channel information of the first and secondwireless nodes.

In another example, a synchronous system for localization of nodes in awireless network architecture includes a first wireless node having awireless device with one or more processing units and RF circuitry fortransmitting and receiving communications in the wireless networkarchitecture including a first RF signal having a first packet. A secondwireless node having a wireless device with one or more processing unitsand RF circuitry to enable bi-directional communications with the firstwireless node in the wireless network architecture includes a second RFsignal having a second packet. The one or more processing units of thefirst wireless node are configured to execute instructions to determinea coarse time of flight estimate of the first and second packets and afine time estimate of the time of flight using channel information ofthe first and second wireless nodes. In one example, the first andsecond wireless nodes have the same reference clock signal. Otherfeatures and advantages of embodiments of the present invention will beapparent from the accompanying drawings and from the detaileddescription that follows below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are illustrated by way of exampleand not limitation in the figures of the accompanying drawings, in whichlike references indicate similar elements, and in which:

FIG. 1 illustrates an exemplar system of wireless nodes in accordancewith one embodiment.

FIG. 2 shows a system with an asymmetric tree and mesh networkarchitecture having multiple hubs for communicating in accordance withone embodiment.

FIG. 3 illustrates a time of flight measurement system in accordancewith one embodiment.

FIG. 4 illustrates a block diagram of a time of flight measurementsystem in accordance with one embodiment.

FIG. 5 illustrates a fully synchronous system that is used for distanceestimation in accordance with one embodiment.

FIG. 6 illustrates how a packet of a recorded RF signal is atime-shifted version of a signal transmitted from device 510 inaccordance with one embodiment.

FIG. 7A illustrates a phase response of an ideal channel in accordancewith one embodiment.

FIG. 7B illustrates a phase response of a non-ideal channel inaccordance with one embodiment.

FIG. 8A shows measured signal amplitude as a function of the delayassociated with exemplary paths for a plot 830 in accordance with oneembodiment.

FIG. 8B illustrates a normalization of amplitude versus ToF delay in aplot 860 in accordance with one embodiment.

FIG. 8C illustrates an asynchronous system that is used for time offlight estimation in accordance with another embodiment.

FIG. 9A illustrates a calibration system having automatic gain controlin accordance with one embodiment.

FIG. 9B illustrates a method for removing hardware delays andnon-idealities using a loopback calibration in accordance with oneembodiment.

FIG. 10 illustrates in one embodiment a 2-way ToF measurement system1000.

FIG. 11 illustrates an asynchronous system 1100 in accordance with analternative embodiment.

FIG. 12 illustrates a method for determining location estimation ofnodes using time of flight techniques in accordance with one embodiment.

FIG. 13A shows an exemplary embodiment of a hub implemented as anoverlay 1500 for an electrical power outlet in accordance with oneembodiment.

FIG. 13B shows an exemplary embodiment of an exploded view of a blockdiagram of a hub implemented as an overlay for an electrical poweroutlet in accordance with one embodiment.

FIG. 14A shows an exemplary embodiment of a hub implemented as a cardfor deployment in a computer system, appliance, or communication hub inaccordance with one embodiment.

FIG. 14B shows an exemplary embodiment of a block diagram of a hub 964implemented as a card for deployment in a computer system, appliance, orcommunication hub in accordance with one embodiment.

FIG. 14C shows an exemplary embodiment of a hub implemented within anappliance (e.g., smart washing machine, smart refrigerator, smartthermostat, other smart appliances, etc.) in accordance with oneembodiment.

FIG. 14D shows an exemplary embodiment of an exploded view of a blockdiagram of a hub 1684 implemented within an appliance (e.g., smartwashing machine, smart refrigerator, smart thermostat, other smartappliances, etc.) in accordance with one embodiment.

FIG. 15 illustrates a block diagram of a sensor node in accordance withone embodiment.

FIG. 16 illustrates a block diagram of a system or appliance 1800 havinga hub in accordance with one embodiment.

DETAILED DESCRIPTION

Systems and methods for precise radio frequency localization in thepresence of multiple communication paths are disclosed herein. In oneexample, an asynchronous system for localization of nodes in a wirelessnetwork architecture includes a first wireless node having a wirelessdevice with one or more processing units and RF circuitry fortransmitting and receiving communications in the wireless networkarchitecture including a first RF signal having a first packet. Thesystem also includes a second wireless node having a wireless devicewith a transmitter and a receiver to enable bi-directionalcommunications with the first wireless node in the wireless networkarchitecture including a second RF signal with a second packet. The oneor more processing units of the first wireless node are configured toexecute instructions to determine a coarse time of flight estimate ofthe first and second packets and a fine time estimate of the time offlight using channel information of the first and second wireless nodes.

In various applications of wireless sensor networks, it may be desirableto determine the location of sensor nodes within the network. Forexample, such information may be used to estimate the relative positionof sensors such as security cameras, motion sensors, temperaturesensors, and other such sensors as would be apparent to one of skill inthe art. This information may then be used to produce augmentedinformation such as maps of temperature, motion paths, and multi-viewimage captures. Therefore, localization systems and methods are desiredto enable accurate, low-power, and context-aware localization of nodesin wireless networks, particularly in indoor environments. For thepurpose of this, indoor environments are also assumed to includenear-indoor environments such as in the region around building and otherstructures, where similar issues (e.g., presence of nearby walls, etc.)may be present.

A wireless sensor network is described for use in an indoor environmentincluding homes, apartments, office and commercial buildings, and nearbyexterior locations such as parking lots, walkways, and gardens. Thewireless sensor network may also be used in any type of building,structure, enclosure, vehicle, boat, etc. having a power source. Thesensor system provides good battery life for sensor nodes whilemaintaining long communication distances.

Embodiments of the invention provide systems, apparatuses, and methodsfor localization detection in indoor environments. U.S. patentapplication Ser. No. 14/830,668 filed on Aug. 19, 2015, which isincorporated by reference herein, discloses techniques for RF-basedlocalization. Specifically, the systems, apparatuses, and methodsimplement localization in a wireless sensor network that primarily usesa tree network structure for communication with periodic mesh-basedfeatures for path length estimation when localization is needed. Thewireless sensor network has improved accuracy of localization whilesimultaneously providing good quality of indoor communication by usinghigh-frequencies for localization and lower frequencies forcommunication.

Tree-like wireless sensor networks are attractive for many applicationsdue to their reduced power requirements associated with the radio signalreception functionality. An exemplar tree-like network architecture hasbeen described in U.S. patent application Ser. No. 14/607,045 filed onJan. 29, 2015, U.S. patent application Ser. No. 14/607,047 filed on Jan.29, 2015, U.S. patent application Ser. No. 14/607,048 filed on Jan. 29,2015, and U.S. patent application Ser. No. 14/607,050 filed on Jan. 29,2015, which are incorporated by reference in entirety herein.

Another type of wireless network that is often used is a mesh network.In this network, communication occurs between one or more neighbors, andinformation may then be passed along the network using a multi-hoparchitecture. This may be used to reduce transmit power requirements,since information is sent over shorter distances. On the other hand,receive radio power requirements may increase, since it is necessary forthe receive radios to be on frequently to enable the multi-hopcommunication scheme.

Based on using the time of flight of signals between nodes in a wirelessnetwork, it is possible to estimate distance between individual pairs ofnodes in a wireless network by exploiting the fact that the speed ofsignal propagation is relatively constant. Embodiments of the presentnetwork architecture allow measuring multiple pairs of path lengths andperforming triangulation and then estimating the relative location ofindividual nodes in three-dimensional space.

FIG. 1 illustrates an exemplar system of wireless nodes in accordancewith one embodiment. This exemplar system 100 includes wireless nodes110-116. The nodes communicate bi-directionally with communications120-130 (e.g., node identification information, sensor data, node statusinformation, synchronization information, localization information,other such information for the wireless sensor network, time of flight(TOF) communications, etc.). Based on using time of flight measurements,path lengths between individual pairs of nodes can be estimated. Anindividual time of flight measurement between nodes 110 and 111 forexample, can be achieved by sending a signal at a known time from node110 to node 111. Node 111 receives the signal, records a time stamp ofreception of the signal of the communications 120, and can then, forexample, send a return signal back to A, with a time stamp oftransmission of the return signal. Node 110 receives the signal andrecords a time stamp of reception. Based on these two transmit andreceive time stamps, an average time of flight between nodes 110 and 111can be estimated. This process can be repeated multiple times and atmultiple frequencies to improve precision and to eliminate or reducedegradation due to poor channel quality at a specific frequency. A setof path lengths can be estimated by repeating this process for variousnode pairs. For example, in FIG. 1, the path lengths are TOF 150-160.Then, by using a geometric model, the relative position of individualnodes can be estimated based on a triangulation-like process.

This triangulation process is not feasible in a tree-like network, sinceonly path lengths between any node and a hub can be measured. This thenlimits localization capability of a tree network. To preserve the energybenefits of a tree network while allowing localization, in oneembodiment of this invention, a tree network for communication iscombined with mesh-like network functionality for localization. Oncelocalization is complete with mesh-like network functionality, thenetwork switches back to tree-like communication and only time offlights between the nodes and the hub are measured periodically.Provided these time of flights are held relatively constant, the networkthen assumes nodes have not moved and does not waste energy isattempting to re-run mesh-based localization. On the other hand, when achange in path length in the tree network is detected, the networkswitches to a mesh-based system and re-triangulates to determinelocation of each node in the network.

FIG. 2 shows a system with an asymmetric tree and mesh networkarchitecture having multiple hubs for communicating in accordance withone embodiment. The system 700 includes a central hub 710 having awireless control device 711, hub 720 having a wireless control device721, hub 782 having a wireless control device 783, and additional hubsincluding hub n having a wireless control device n. Additional hubswhich are not shown can communicate with the central hub 710, otherhubs, or can be an additional central hub. Each hub communicatesbi-directionally with other hubs and one or more sensor nodes. The hubsare also designed to communicate bi-directionally with other devicesincluding device 780 (e.g., client device, mobile device, tablet device,computing device, smart appliance, smart TV, etc.).

The sensor nodes 730, 740, 750, 760, 770, 788, 792, n, and n+1 (orterminal nodes) each include a wireless device 731, 741, 751, 761, 771,789, 793, 758, and 753, respectively. A sensor node is a terminal nodeif it only has upstream communications with a higher level hub or nodeand no downstream communications with another hub or node. Each wirelessdevice includes RF circuitry with a transmitter and a receiver (ortransceiver) to enable bi-directional communications with hubs or othersensor nodes.

In one embodiment, the central hub 710 communicates with hubs 720, 782,hub n, device 780, and nodes 760 and 770. These communications includecommunications 722, 724, 774, 772, 764, 762, 781, 784, 786, 714, and 712in the wireless asymmetric network architecture. The central hub havingthe wireless control device 711 is configured to send communications toother hubs and to receive communications from the other hubs forcontrolling and monitoring the wireless asymmetric network architectureincluding assigning groups of nodes and a guaranteed time signal foreach group.

The hub 720 communicates with central hub 710 and also sensors nodes730, 740, and 750. The communications with these sensor nodes includecommunications 732, 734, 742, 744, 752, and 754. For example, from theperspective of the hub 720, the communication 732 is received by the huband the communication 734 is transmitted to the sensor node. From theperspective of the sensor node 730, the communication 732 is transmittedto the hub 720 and the communication 734 is received from the hub.

In one embodiment, a central hub (or other hubs) assign nodes 760 and770 to a group 716, nodes 730, 740, and 750 to a group 715, nodes 788and 792 to a group 717, and nodes n and n+1 to a group n. In anotherexample, groups 716 and 715 are combined into a single group.

By using the architectures illustrated in FIGS. 1-2, nodes requiringlong battery life minimize the energy expended on communication andhigher level nodes in the tree hierarchy are implemented using availableenergy sources or may alternatively use batteries offering highercapacities or delivering shorter battery life. To facilitate achievementof long battery life on the battery-operated terminal nodes,communication between those nodes and their upper level counterparts(hereafter referred to as lowest-level hubs) may be established suchthat minimal transmit and receive traffic occurs between thelowest-level hubs and the terminal nodes.

In one embodiment, the nodes spend most of their time (e.g., more than90% of their time, more than 95% of their time, approximately 98% ormore than 99% of their time) in a low-energy non-communicative state.When the node wakes up and enters a communicative state, the nodes areoperable to transmit data to the lowest-level hubs. This data mayinclude node identification information, sensor data, node statusinformation, synchronization information, localization information andother such information for the wireless sensor network.

To determine the distance between two objects based on RF, rangingmeasurements are performed (i.e., RF communication is used to estimatethe distance between the pair of objects). To achieve this, an RF signalis sent from one device to another. FIG. 3 illustrates a time of flightmeasurement system in accordance with one embodiment. A transmittingdevice 310 sends an RF signal 312, and a receiving device 320 receivesthe RF signal 312, as shown in FIG. 3. Here, in an exemplary wirelessnetwork, the device 310 may be a hub or a node, and the device 320 mayalso be a hub or a node.

FIG. 4 illustrates a block diagram of a time of flight measurementsystem in accordance with one embodiment. A receiving device (e.g.,device 320) receives the transmission from the transmitting device(e.g., device 310) and processes the RF signal 412 to generate at leastone coarse estimation 442 using a coarse resolution estimator 440 and atleast one fine estimation 452 of the propagation delay between the twodevices over the air using a fine resolution estimator 450. The system400 then utilizes a combiner 460 to combine the coarse time estimation442 and the fine time estimation 452 to generate an accuratetime-of-flight measurement 470. This time-of-flight measurement 470 canthen be multiplied by the speed of light to calculate the distance, asshown in FIG. 4.

Time of flight measurements are inherently sensitive to the timing ofoperations within the network, and therefore, the clocking of thedevices performing the measurements is important. FIG. 5 illustrates afully synchronous system that is used for distance estimation inaccordance with one embodiment. In a fully synchronous system 500, i.e.both devices share the same clock reference, device 510 first sends a RFsignal 512 (e.g., RF signal having a packet) to device 520 at time T1.The packet arrives at the device 520 at time T2 and triggers a packetdetection algorithm in device 520 to register this time T2. Because itis a synchronous system, the coarse estimation of time of flight can becalculated as T2−T1. However, the resolution of this measurement islimited by the time resolution of the sampling clock, which will have afrequency of f_(s) and a time resolution of Ts. The time resolution isillustrated in FIG. 6. Here, the sampling clock represents the maximumprecision of time estimation of the system, and may, in an exemplarysystem, be set by the frequency of the clock used to control thecircuitry used to detect the timing of transmission or reception. Forexample, if the sampling clock is 100 MHz, the resolution of thismeasurement will be at 10 nanoseconds (ns), which corresponds to roughly10-foot accuracy.

In order to improve this accuracy, the RF signal 512 may be recorded andanalyzed at device 520. FIG. 6 illustrates how a packet of a recorded RFsignal is a time-shifted version of a signal transmitted from device 510in accordance with one embodiment. At a sample clock time interval (Ts),a packet 514 of the RF signal 512 is detected at T2. The true beginningof the received packet 514 is a fraction period (e.g., ΔT) of a samplingclock period earlier than T2.

Multiple methods can be used to estimate this fractional period (e.g.,ΔT). For example, the time domain signal can be converted into frequencydomain using a fast fourier transform (FFT), then divided by thespectrum of the original signal to obtain the frequency response of thechannel. In an orthogonal frequency-division multiplexing (OFDM) basedsystem, this information can also be obtained from channel senseinformation (CSI). In an ideal channel over the air, the channelresponse in frequency domain isH(f)=Ae ^(j2πfΔT)

Where A is the loss of the channel and ΔT is the delay of the channel.FIG. 7A illustrates a phase response of an ideal channel in accordancewith one embodiment. A plot of phase 202 on a vertical axis andfrequency 204 on a horizontal axis illustrates an ideal channel 210 as astraight line with a slope corresponding to 2π*ΔT.

Combining ΔT with T2−T1, an accurate distance estimation can beestablished as:Distance=(T2−T1−slope/(2π))XC

-   -   where C is the speed of light.

In the case of non-ideal channels, there are multiple reflections fromthe environment and the overall channel response can be annotated asH(f)=ΣA _(k) e ^(j2πfΔTk)

where A_(k) is the amplitude of each path, and ΔT_(k) is the delay ofeach path. As a result, the channel response will differ from a straightline in phase. FIG. 7B illustrates a phase response of a non-idealchannel in accordance with one embodiment. A plot of phase 252 on avertical axis and frequency 254 on a horizontal axis illustrates anon-ideal channel 260.

Advanced algorithms like Matrix Pencil, MUSIC, etc. can be used toestimate the minimum delay of the multiple paths (ΔT_(k)), and thedistance can be calculated from this extracted minimum delay.Distance=(T2−T1−S{H(f)})XC

-   -   where S{H(f)} is the result of minimum delay extraction from the        channel response measurement, i.e. it should be equal to        min{ΔT_(k)}.

By separating the system into coarse and fine estimation, highefficiency and high performance can be achieved simultaneously. Thecoarse time estimator can cover long range, albeit with reducedaccuracy. Such low accuracy requirements also make this estimator lesssensitive to interference and multi-path, which is an important errorsource for time-of-flight measurements. There are multiple methods thatcan be used to determine the coarse time estimation. For example, thecoarse time can be extracted from timestamps that indicate the time whenthe signal is transmitted and when the signal is received.Alternatively, a measurement of the phase of signals received atmultiple carrier frequencies can be unwrapped using the ChineseRemainder Theorem to estimate the coarse delay. A non-uniform discreteFourier transform using a particular set of non-uniform carrierfrequencies can also be used to estimate the coarse delay.

On the other hand, the fine resolution estimator only needs to cover arelatively short range, therefore reducing the computing resourcesneeded for the system. The fine estimation is only required to cover amaximum delay of one coarse sample period. Advanced algorithms can alsobe applied to this estimator to improve the performance withinterference and multi-path environments. This fine estimate can also bederived using multiple methods. For example, it can be derived fromcross correlation of the received signal with an ideal version of thesignal. It can also be derived from channel estimation using thereceived signal. The channel estimate can be converted into a fine delayestimate by using the slope of the phase, inverse FFT, matrix pencil,MUSIC, or other methods.

In linear algebra, matrix pencil is defined as a matrix-valued functionwith a complex variable λL(λ)=Σλ^(i) A _(i)

In the context of ranging measurement, the channel response has asimilar format ofH(f)=H(n*fsub)=ΣA _(k) e ^(j2πfΔTk) =ΣA _(k)(e ^(j2πfsubΔTk))^(n) =ΣA_(k)(λ)^(n)

-   -   where frequency domain measurement is performed at frequencies        equally spaced by fsub (sub-carrier frequency).

The matrix-pencil method can therefore be used to extract the poles (λ)of such system. Once all possible poles (λ_(k)) are extracted frommeasurement, each time delay can be calculated asΔT _(k)=log(λ_(k))/(j2πf _(sub))

In another embodiment, a multiple signal classification (MUSIC)algorithm may be used. MUSIC is based on signal modeling consisting of asum of harmonic signalsX(n)=ΣA _(k) e ^(j*wk*n)

Similar to the case of Matrix-Pencil, the channel response can bewritten asH(f)=H(n*fsub)=ΣA _(k) e ^(j*2πf*ΔTk) =ΣA _(k) e ^(j*2π*fsub*ΔTk*n)

The algorithm then extracts A_(k) and w_(k) based on the measurementresults X(n), and the delay elements can be calculated asΔT _(k) =w _(k)/(2πf _(sub)))

In the systems described herein, noise, numerical errors, and other suchlimitations may cause the wrong delay to be estimated. If longer-thanthe actual time delay is estimated, the time delay result will not beaffected because only the shortest delay is used for distance-relateddelay calculation. On the other hand, if a shorter-than time of flightdelay is estimated, it can be mistaken as the actual time of flightdelay. Therefore, it is important to eliminate false short paths toimprove the time delay estimation accuracy. Therefore, in oneembodiment, a system to correct for this error is implemented.

In a wireless environment, the amplitude of the signal decreases withdistances quadratically as described by free-space-path-loss. Therefore,the shorter path estimated from the delay-estimation algorithm isexpected to have a higher amplitude. This foreknowledge is then used toeliminate false short path estimates. The amplitude of the receivedsignals can be normalized by multiplying the square of the estimateddistance by the estimated amplitude. If this normalized amplitude islower than a certain threshold, it is an indication that the estimationof this path is due to either noise or algorithm limitations, andtherefore can be eliminated.

In practice, the actual signal strength also depends on the additionalloss incurred on the path, including walls, windows, reflections, andetc. The threshold mentioned previously can be set according to theexpected loss due to these factors, or can be set according to empiricaldata.

In one example, the path estimation algorithm may produce 5 paths. Oneof these paths may be generated due to noise and have a ToF delayestimate of 20 ns when the actual line of sight (LOS) delay is 40 ns.This is shown in FIGS. 8A and 8B. FIG. 8A shows the measured signalamplitude as a function of the delay associated with the 5 exemplarypaths for a plot 830 in accordance with one embodiment. The plot 830illustrates ToF delay (ns) 842 for 5 paths on a horizontal axis andmeasured signal amplitude (dB) 840 on a vertical axis. The data points851-855 represent measured signal amplitudes for different ToF delays ofthe 5 paths. The data point 851 represents a potential false short pathdue to having a ToF delay of 20 ns that is less than the actual LOSdelay of 40 ns. The LOS amplitude could be 1 and the amplitude of thefalse short path that is associated with the data point 851 could be0.1. The amplitudes can be normalized by the square of the distance sothe LOS amplitude is 1 and the normalized amplitude of the false path is0.025. Such a normalization of amplitude versus ToF delay is shown in aplot 860 of FIG. 8B in accordance with one embodiment. The plot 860illustrates ToF delay (ns) 862 for 5 paths on a horizontal axis and thenormalization of the signal amplitude (dB) 864 on a vertical axis. Thedata points 871-875 represent normalization of the measured amplitudesfor different ToF delays of the 5 paths. The data point 871 represents apotential false short path due to having a ToF delay of 20 ns that isless than the actual LOS delay of 40 ns. The normalized amplitude of thefalse path that is associated with the data point 871 is 32 dB below theLOS amplitude. In one example, if a maximum of 30 dB of loss fromenvironmental factors (excluding path loss) is expected, then the 20 nspath that is associated with the data point 871 can be eliminated as afalse short path estimate.

The threshold itself can also be a function of path length to accountfor the amount of environmental loss expected in a short distance versusa long distance. Other implementations could also incorporate thedynamic range of the physical hardware (e.g., dynamic operating range ofsignal levels of RF receivers of hubs, sensor nodes, etc.) in settingthe threshold.

In another embodiment, an asynchronous system is used for time of flightestimation. In the case where the two devices are asynchronous, timingoffsets between devices can introduce large errors into the delayestimate. The aforementioned setup can be extended into a two-way systemto mitigate this issue. FIG. 8C illustrates an asynchronous system thatis used for time of flight estimation in accordance with anotherembodiment. A device 810 first sends a RF signal 812 having a packet todevice 820 at time T1. The packet arrives at the device 820 at time T2,triggering the packet detection algorithm in device 820 to register thistime. Device 820 then sends back a signal 822 having a packet at timeT3, which arrives at device 810 at time T4 and triggers the device 810to register the time and process the waveform. Notice that unlike thecase of fully synchronous system, T1 and T4 are times recorded on device810 and therefore is referenced to its reference clock. T2 and T3 arerecorded based on the time reference of device 820. The coarse timeestimation is done as2×ToF=(T4−T1)−(T3−T2)

Because T4 and T1 are sampled at the same clock, there is no arbitraryphase between T4 and T1. Therefore, T4−T1 is accurate in time; the sameprinciple applies to T3−T2. Therefore, this measurement is immune to anyphase-walking between the two devices resulting from the asynchronousnature of this system. Similar to the previous embodiment, thismeasurement is limited by the resolution of the sampling clock period ofT1/T2/T3/T4. In order to improve this accuracy, a frequency responsemeasurement can be performed on both devices. Device 820 measures thechannel response using the packet from device 810 and device 810measures the channel response using the packet from device 820. Becausethe two devices are not synchronous, there is an uncertainty in thephase between the two clocks, annotated as T_(offset) here. This phaseoffset of the clock manifests itself as an extra phase of the channelresponse measurement on each side, but it can be eliminated bymultiplying the channel responses from the two sides. Assuming thechannel response is the same as before, then the measurement from device820 will beH ₈₂₀(f)=H(f)e ^(−j2πfToffset)

The measurement from the device 810 will beH ₈₁₀(f)=H(f)e ^(+j2πfToffset)

The combined channel response is thereforeH ₈₁₀(f)H ₈₂₀(f)=H(f)²=(ΣA _(k) e ^(−j2πfΔTk))²

which cancels out the phase difference between the two clocks. Similarto the previous embodiment, algorithms such as matrix pencil, MUSIC,etc. can be used to estimate the delay from H₈₁₀(f) H₈₂₀(f), whichproduces the 2 min{ΔT_(k)}, and the distance measurement is given byDistance=[(T4−T1)/2−(T3−T2)/2−S{H ₈₁₀(f)H ₈₂₀(f)}/2]×C

Alternatively T_(offset) can be estimated fromH ₈₁₀(f)/H ₈₂₀(f)=e ^(+2j2πfToffset)

The T_(offset) is half the phase slope of the divided channel responses.The channel response in either direction can be corrected by thecalculated offset. The distance estimation can then be calculated asDistance=[(T4−T1)/2−(T3−T2)/2−S{H ₈₁₀(f)}−T _(offset)]×COrDistance=[(T4−T1)/2−(T3−T2)/2−S{H ₈₂₀(f)}±T _(offset)]×C

This method has advantages over the multiplication method. The H(f)²channel response includes terms at double the amplitude and distance ofeach path as well as cross terms for every 2 path permutation. That isfor a 2 path case, A₈₁₀ ²e^(j 2πf2 ΔT1). A₈₂₀ ²e^(j 2πf2 ΔT2), andA₈₁₀A₈₂₀e^(j 2πf (ΔT1+ΔT2)). The fine estimation methods are moreeffective and more robust to noise when applied to the unidirectionalchannel response H(f) as there are less paths to distinguish and lowerdynamic range.

The channel estimate can also be combined with the coarse delay estimatebefore estimating the total path delays. The coarse delay can be addedto the channel estimate as a linear phase shift. In the synchronous caseit is:T _(coarse)=½(T ₄ −T ₁)−(T ₃ −T ₂))H _(tot)(f)=H(f)e ^(−j2πfT) ^(coarse)

In the case of an asynchronous system, the calculated clock phase offsetis also applied as an addition or subtraction of a linear phase shift tothe forward or reverse channel estimate.H _(tot)(f)² =H(f)² e ^(−j2πf2T) ^(coarse)OrH _(tot)(f)=H _(forw)(f)e ^(−j2πfT) ^(coarse) e ^(j2πfT) ^(offset)OrH _(tot)(f)=H _(rev)(f)e ^(−j2πfT) ^(coarse) e ^(−j2πfT) ^(offset)

Then matrix pencil, MUSIC, or other methods can be applied to the coarseplus fine channel estimate. This allows all of the estimated paths to bereal distances relative to 0 distance. This aids in the elimination offalse short paths and the selection of the line of sight path.

If the nodes have synchronous clocks, then the channel estimate (with orwithout the coarse delay correction) can be averaged across multiplepackets or multiple wireless transmissions. This averaging can beperformed before using matrix pencil, MUSIC, or other methods for pathestimation. Averaging of the channel estimate improves the signal tonoise ratio (SNR) as long as the multiple channel estimates are measuredquickly relative to changes in the channel (the channel coherence time).Higher SNR improves the accuracy of the path estimation and allowsweaker paths to be distinguished from noise.

In an asynchronous system, the phase offset correction methods describedabove also allow averaging multiple channel estimates. When using themultiplication method the Htot(f)² can be averaged across multipletransmissions. When using the division method, Htot(f) can be averagedacross multiple transmissions.

The aforementioned short path elimination algorithm can also be used inasynchronous systems such as those disclosed above.

As is apparent from the above embodiments, measurement of timing iscritical to establishment of distance estimation. Errors in timing canreduce accuracy of distance estimation. Timing errors often exist withinwireless systems. For example, automatic gain control (AGC) is commonlyused to ensure robust receiver operation for signals of varying signalstrength. During operation, AGC stages may have delays that vary basedon the gain. As such, these variations in delay can add to theuncertainty of TOF estimation. In one embodiment, this error can beminimized through calibration. The delay as a function of AGC stage gainmay be pre-measured and used to correct the timing during the actual TOFmeasurement, by subtracting such deviations from the baseline delay.FIG. 9A illustrates RF circuitry having automatic gain control inaccordance with one embodiment. The RF circuitry 900 (e.g., 1550, 1670,1692, 1770, 1870, etc.) can be included in any wireless node (e.g., hub,sensor node) as described in embodiments of the present disclosure. TheRF circuitry 900 includes a low noise amplifier to receive a RF signaland to generate an amplified signal sent to an in-phase quadrature (I/Q)downconversion unit 920 to downconvert RF signals to a desiredintermediate frequency. A variable gain amplifier 930 amplifiers theintermediate frequency signal and then an analog to digital converter(ADC) converts the amplified signal into a baseband signal. The AGC 950is a closed-loop feedback regulating circuit that provides a controlledsignal amplitude at its output 952 despite variation of the amplitude inits input 942. As discussed above, the delay as a function of AGC stagegain (e.g., AGC 950) may be pre-measured and used to correct the timingduring the actual TOF measurement, by subtracting such deviations from abaseline pre-measured delay. Similarly, other calibrated systemconfigurations such as filter delay may be pre-measured and deducted aswell.

FIG. 9B illustrates a method for removing hardware delays andnon-idealities using a loopback calibration in accordance with oneembodiment. The operations of this method may be executed by a wirelessdevice, a wireless control device of a hub (e.g., an apparatus), asystem, or a remote device or computer which includes processingcircuitry or processing logic. The processing logic may include hardware(circuitry, dedicated logic, etc.), software (such as is run on ageneral purpose computer system or a dedicated machine or a device), ora combination of both. In one embodiment, an anchor node, a hub, awireless device, or a remote device or computer performs the operationsof this method. Algorithms associated with the various computations maybe performed in a remote computer to which the relevant data associatedwith distance measurements is sent. The remote device or computer may beat a different location (e.g., different network, in the cloud) than thewireless network architecture having a plurality of wireless nodes. Theremote device or computer may be at a different location within thewireless network architecture than certain nodes that are transmittedand receiving communications within the wireless network architecture.

During a manufacturing process of components of a wireless networkarchitecture or upon initialization of the wireless networkarchitecture, at operation 980, the processing logic calibrates at leastone component of RF circuitry (e.g., transmit chain, receive chain,etc.) of a wireless device or sensor node that has a delay. Thecalibration of at least one component may include generating a firstloopback calibration signal and passing this signal through a RFtransmit chain of a first node (e.g., wireless device or sensor node) atoperation 981. This first loopback calibration signal is not necessarilytransmitted over the air. The transmit chain is connected to a receivechain of the wireless device or sensor node explicitly or through onchip leakage. This connection can be performed at any point in the RFchain. In one example, the RF transmit and receive chains are set to beactive simultaneously. The blocks of the RF transmit chain that thefirst loopback calibration signal travels through before looping backthrough the RF receive chain can be characterized and calibrated forlater measurements. At operation 982, the processing logic measures afirst transmit time delay T_(tx1) for passing the first loopbackcalibration signal through the transmit chain and also a first receivetime delay T_(rx1) for passing the first loopback calibration signalthrough the receive chain of the first node (e.g., wireless device,sensor node). At operation 984, the processing logic generates a secondloopback calibration signal, passes the second signal through transmitand receive chains of a second node, and measures a second transmit timedelay T_(tx2) for passing the second loopback calibration signal througha transmit chain of a second node and also a second receive time delayT_(rx2) for passing the second loopback calibration signal through thereceive chain of the second node (e.g., wireless device, sensor node).

At operation 986, the processing logic can calculate a time delay of thefirst loopback calibration signal based on the first transmit time delayand the first receive time delay (e.g., T_(lb1)=T_(tx1)+T_(rx1) for thefirst node) and also a time delay of the second calibration signal iscalculated based on the second transmit time delay and the secondreceive time delay (e.g., T_(lb2)=T_(tx2)+T_(rx2) for the second node).Then at operation 988 the measured RTT for communications transmittedbetween the first and second nodes can be corrected based on the T_(lb1)and T_(1b2) (e.g., RTT_(ota)=RTT_(meas)−T_(lb1)−T_(lb2)).

At operation 990, the processing logic calibrates the frequency responseof the hardware (e.g., transmit chain, receive chain) for the first andsecond nodes. In one example, the loopback calibration can provideH_(lb1)=H_(tx1)*H_(rx1) for the first node and H_(lb2)=H_(tx2)*H_(rx2)for the second node. The loopback calibration further providesH_(ota2)(f)=H_(meas2)(f)/(H_(lb1)*H_(lb2)). In the unidirectional case,the loopback calibration from just the receive side, just the transmitside, or some combination of the two (average, etc) can be used. Forexample, H_(ota)(f)=H_(meas)(f)/H_(lb2).

The loopback can be done as part of production or factory testing andresults stored in memory. Alternatively, the loopback can be performedperiodically during normal operation. Performing periodically duringnormal operation allows changes in hardware delays and non-idealitiesover time, temperature, etc to be calibrated and correctedautomatically. In the production or factory test method, variations overtemperature would need to be characterized. An on chip or on-boardtemperature sensor could be used to adjust the calibration correctiondepending on the characterized temperature profile.

In one embodiment, the baseband hardware allows full duplex operation.In this case, the signal can be transmitted and then immediatelyreceived after going through the transmit and receive chains. The CSI,transmit, and receive timestamps are combined to generate a calibrationprofile for the hardware. The delay can be extracted and removeddirectly or the frequency response of the hardware itself can be removedfrom subsequent measurements.

In another embodiment, the baseband is not full duplex. In this case thetransmit packet can be generated independently from the baseband. Theraw data for a packet can be stored in memory and read out directly to adigital-to-analog converter (DAC). It goes through the transmit (TX) andreceive (RX) RF chains as before. The baseband is set to be in receivemode and can capture the CSI and timestamp of the incoming packet. TheCSI, transmit, and receive timestamps can be used as in the previouscase. The transmit timestamp needs to be captured at the time the memoryreadout to the DAC is triggered.

As shown above, in an asynchronous system, the information from the twodevices needs to be combined for calculation. In order to do that, inone embodiment, one of the devices can send the information to the otherdevice, either using the same RF signals (e.g., 812, 822, 1022, 1023)mentioned before, or using an independent RF signal path 1024, as shownin FIG. 10 in one embodiment of a 2-way ToF measurement system 1000.

FIG. 11 illustrates an asynchronous system 1100 in accordance with analternative embodiment. Devices 1110 and 1120 can send their informationto a third device 1130 through the same RF signals 1112 and 1113 orindependent RF signals 1122 and 1123, and the third device 1130 canprocess and combine the information to calculate time-of-flight.

Once distances between the various pairs on the networks areestablished, the information can be passed to one or more members of thenetwork or even to systems outside the network for estimation ofrelative and/or absolute locations of the various members of thenetwork. This can be performed using a variety of techniques. Forexample, triangulation approaches may be used as are well known to thoseof skill in the art. Error minimization techniques such as least squaresapproaches may be used to improve accuracy and reduce errors of positionestimation. Such approaches may be used to reduce any errors associatedwith distance estimation in the embodiments above by taking advantage ofthe redundant information produced in the various paired distanceestimates. Other techniques that may be used to perform localizationbased on the determined ranging data include multi-dimensional scaling,self-positioning algorithm, terrain algorithm, collaborativemultilateration, distributed maximum likelihood, hyperbolic positionfix, mobile geographic distributed localization, elastic localizationalgorithm, and other such anchor-free and anchor-based localizationalgorithms.

The localization information determined herein may be used to facilitateor improve the operation of a wireless sensor network. An exemplarywireless sensor network is disclosed in U.S. patent application Ser. No.14/925,889 filed on Aug. 19, 2015, which is incorporated by referenceherein. The localization may be used to establish logical and/orfunctional relationships within the network. In one exemplaryembodiment, localization information may be used to define constellationmembership in a sensor network that allows node-to-node communicationwith a constellation of a normally tree-like network, such as shown inFIG. 2. In this embodiment, localization is used to identify nodes thatshould be assigned to be within the same constellation. These may thencommunicate directly with each other without going through the hub. Anadvantage of this approach is that errors in distance estimate thatsurvive triangulation calculations will not cause catastrophic failureof the network; rather, at most, these errors will cause erroneousassignment of constellations. Use of overlapping constellationassignments or loose constellation assignment rules may further be usedto prevent this from impacting network performance in an undesirablemanner.

In one embodiment, a Location Algorithm includes anchor-basedTriangulation. In an anchor-based system, the location of anchor nodes(e.g., hubs, sensors, devices, etc.) is known. The unknown location ofother devices is calculated based on the known location of the anchorsas well as the measured distance between each device and each anchor.The location of these unknown devices is calculated one by one with thesame procedure. For each of the devices, the distance measurement withanchor i is:d _(i)=√{square root over ((x−x _(i))²+(y−y _(i))²+(z−z _(i))²)}+n _(i)

where x_(i), y_(i), and z_(i) are the coordinates of the ith anchor;d_(i) is the measured distance between the unknown device and the ithanchor; x, y, and z are the coordinates of the unknown device, which isthe goal of the estimation. By setting up different error function forthe estimation, one can use linear least squares to calculate thelocation of the unknown device (x,y,z).

In another embodiment, for an Anchor-less Triangulation setup, there isno known location for any of the devices. The algorithm has to use thedistance measurement between pairs of the devices to determine therelative location of each device. The goal is to find out the relativelocation of all devices to minimize the overall error of the distancemeasurement. There are multiple types of algorithms including anincremental algorithm and a concurrent algorithm. An incrementalalgorithm starts with a small set of devices and calculates theirlocations based on distance measurement. The small set of device is thenused as anchor nodes for other devices. It is a simple algorithm butwith the drawback that the error in early calculated nodes can be easilypropagated to the later nodes, even with an advanced algorithm whichupdates location of the early nodes.

A concurrent algorithm solves the issue of the incremental algorithm dueto the concurrent algorithm estimating all locations at the same time toachieve a global optimum with lower error than incremental algorithm. Itusually uses iterative process to update the location of the devices,therefore it will take longer time to converge while using morecomputation power and memory.

FIG. 12 illustrates a method for determining location estimation ofnodes using time of flight techniques in accordance with one embodiment.The operations of method 1200 may be executed by a wireless device, awireless control device of a hub (e.g., an apparatus), a system, or aremote device or computer which includes processing circuitry orprocessing logic. The processing logic may include hardware (circuitry,dedicated logic, etc.), software (such as is run on a general purposecomputer system or a dedicated machine or a device), or a combination ofboth. In one embodiment, an anchor node, a hub, a wireless device, or aremote device or computer performs the operations of method 1200.Algorithms associated with the various computations may be performed ina remote computer to which the relevant data associated with distancemeasurements is sent. The remote device or computer may be at adifferent location (e.g., different network, in the cloud) than thewireless network architecture having a plurality of wireless nodes. Theremote device or computer may be at a different location within thewireless network architecture than certain nodes that are transmittedand receiving communications within the wireless network architecture.

Upon initialization of a wireless network architecture, at operation1201, the processing logic calibrates at least one component (e.g.,automatic gain control (AGC) stage of the RF circuitry, filter stage ofthe RF circuitry, etc.) that has a delay. The calibration of at leastone component may include measuring a delay of the at least onecomponent (e.g., AGC stage as a function of gain, filter stage),determining if a deviation exists between the measured delay and abaseline delay of the at least one component, and correcting a timing ofthe determined time of flight estimate if a deviation exists. Thecalibration typically occurs during the initialization of the wirelessnetwork architecture. Alternatively, the calibration may occur at alater time of the method 1200.

At operation 1202, the hub (or anchor node, wireless device, remotedevice or computer) having radio frequency (RF) circuitry and at leastone antenna transmits communications to a plurality of sensor nodes inthe wireless network architecture (e.g., wireless asymmetric networkarchitecture). At operation 1203, the RF circuitry and at least oneantenna of the hub (or anchor node, wireless device, remote device orcomputer) receives communications from the plurality of sensor nodeseach having a wireless device with a transmitter and a receiver toenable bi-directional communications with the RF circuitry of the hub inthe wireless network architecture. At operation 1205, processing logicof a hub (or anchor node, wireless device, remote device or computer)having a wireless control device initially causes a wireless network ofsensor nodes to be configured as a mesh-based network architecture for atime period (e.g., predetermined time period, time period sufficient forlocalization, etc.).

At operation 1206, the processing logic of the hub (or anchor node,wireless device, remote device or computer) determines localization ofat least two nodes (or all nodes) using at least one time of flighttechnique and possibly a signal strength technique as discussed in thevarious embodiments disclosed herein.

At operation 1208, upon localization of the at least two network sensornodes being complete, the processing logic of the hub (or anchor node,wireless device, remote device or computer) terminates time of flightmeasurements if any time of flight measurements are occurring andcontinues monitoring the signal strength of communications with the atleast two nodes. Similarly, the at least two nodes may monitor thesignal strength of communications with the hub. At operation 1210, theprocessing logic of the hub (or anchor node, wireless device, remotedevice or computer) configures the wireless network in a tree based ortree-like network architecture (or tree architecture with no mesh-basedfeatures) upon completion of localization.

The communication between hubs and nodes as discussed herein may beachieved using a variety of means, including but not limited to directwireless communication using radio frequencies, Powerline communicationachieved by modulating signals onto the electrical wiring within thehouse, apartment, commercial building, etc., WiFi communication usingsuch standard WiFi communication protocols as 802.11a, 802.11b, 802.11n,802.11ac, and other such Wifi Communication protocols as would beapparent to one of ordinary skill in the art, cellular communicationsuch as GPRS, EDGE, 3G, HSPDA, LTE, and other cellular communicationprotocols as would be apparent to one of ordinary skill in the art,Bluetooth communication, communication using well-known wireless sensornetwork protocols such as Zigbee, and other wire-based or wirelesscommunication schemes as would be apparent to one of ordinary skill inthe art.

The implementation of the radio-frequency communication between theterminal nodes and the hubs may be implemented in a variety of waysincluding narrow-band, channel overlapping, channel stepping,multi-channel wide band, and ultra-wide band communications.

The hubs may be physically implemented in numerous ways in accordancewith embodiments of the invention. FIG. 13A shows an exemplaryembodiment of a hub implemented as an overlay 1500 for an electricalpower outlet in accordance with one embodiment. The overlay 1500 (e.g.,faceplate) includes a hub 1510 and a connection 1512 (e.g.,communication link, signal line, electrical connection, etc.) thatcouples the hub to the electrical outlet 1502. Alternatively (oradditionally), the hub is coupled to outlet 1504. The overlay 1500covers or encloses the electrical outlets 1502 and 1504 for safety andaesthetic purposes.

FIG. 13B shows an exemplary embodiment of an exploded view of a blockdiagram of a hub 1520 implemented as an overlay for an electrical poweroutlet in accordance with one embodiment. The hub 1520 includes a powersupply rectifier 1530 that converts alternating current (AC), whichperiodically reverses direction, to direct current (DC) which flows inonly one direction. The power supply rectifier 1530 receives AC from theoutlet 1502 via connection 1512 (e.g., communication link, signal line,electrical connection, etc.) and converts the AC into DC for supplyingpower to a controller circuit 1540 via a connection 1532 (e.g.,communication link, signal line, electrical connection, etc.) and forsupplying power to RF circuitry 1550 via a connection 1534 (e.g.,communication link, signal line, electrical connection, etc.). Thecontroller circuit 1540 includes memory 1542 or is coupled to memorythat stores instructions which are executed by processing logic 1544(e.g., one or more processing units) of the controller circuit 1540 forcontrolling operations of the hub for forming, monitoring, andperforming localization of the wireless asymmetrical network asdiscussed herein. The RF circuitry 1550 may include a transceiver orseparate transmitter 1554 and receiver 1556 functionality for sendingand receiving bi-directional communications via antenna(s) 1552 with thewireless sensor nodes. The RF circuitry 1550 communicatesbi-directionally with the controller circuit 1540 via a connection 1534(e.g., communication link, signal line, electrical connection, etc.).The hub 1520 can be a wireless control device 1520 or the controllercircuit 1540, RF circuitry 1550, and antenna(s) 1552 in combination mayform the wireless control device as discussed herein.

FIG. 14A shows an exemplary embodiment of a hub implemented as a cardfor deployment in a computer system, appliance, or communication hub inaccordance with one embodiment. The card 1662 can be inserted into thesystem 1660 (e.g., computer system, appliance, or communication hub) asindicated by arrow 1663.

FIG. 14B shows an exemplary embodiment of a block diagram of a hub 1664implemented as a card for deployment in a computer system, appliance, orcommunication hub in accordance with one embodiment. The hub 1664includes a power supply 1666 that provides power (e.g., DC power supply)to a controller circuit 1668 via a connection 1674 (e.g., communicationlink, signal line, electrical connection, etc.) and provides power to RFcircuitry 1670 via a connection 1676 (e.g., communication link, signalline, electrical connection, etc.). The controller circuit 1668 includesmemory 1661 or is coupled to memory that stores instructions which areexecuted by processing logic 1663 (e.g., one or more processing units)of the controller circuit 1668 for controlling operations of the hub forforming, monitoring, and performing localization of the wirelessasymmetrical network as discussed herein. The RF circuitry 1670 mayinclude a transceiver or separate transmitter 1675 and receiver 1677functionality for sending and receiving bi-directional communicationsvia antenna(s) 1678 with the wireless sensor nodes. The RF circuitry1670 communicates bi-directionally with the controller circuit 1668 viaa connection 1672 (e.g., communication link, signal line, electricalconnection, etc.). The hub 1664 can be a wireless control device 1664 orthe controller circuit 1668, RF circuitry 1670, and antenna(s) 1678 incombination may form the wireless control device as discussed herein.

FIG. 14C shows an exemplary embodiment of a hub implemented within anappliance (e.g., smart washing machine, smart refrigerator, smartthermostat, other smart appliances, etc.) in accordance with oneembodiment. The appliance 1680 (e.g., smart washing machine) includes ahub 1682.

FIG. 14D shows an exemplary embodiment of an exploded view of a blockdiagram of a hub 1684 implemented within an appliance (e.g., smartwashing machine, smart refrigerator, smart thermostat, other smartappliances, etc.) in accordance with one embodiment. The hub includes apower supply 1686 that provides power (e.g., DC power supply) to acontroller circuit 1690 via a connection 1696 (e.g., communication link,signal line, electrical connection, etc.) and provides power to RFcircuitry 1692 via a connection 1698 (e.g., communication link, signalline, electrical connection, etc.). The controller circuit 1690 includesmemory 1691 or is coupled to memory that stores instructions which areexecuted by processing logic 1688 (e.g., one or more processing units)of the controller circuit 1690 for controlling operations of the hub forforming, monitoring, and performing localization of the wirelessasymmetrical network as discussed herein. The RF circuitry 1692 mayinclude a transceiver or separate transmitter 1694 and receiver 1695functionality for sending and receiving bi-directional communicationsvia antenna(s) 1699 with the wireless sensor nodes. The RF circuitry1692 communicates bi-directionally with the controller circuit 1690 viaa connection 1689 (e.g., communication link, signal line, electricalconnection, etc.). The hub 1684 can be a wireless control device 1684 orthe controller circuit 1690, RF circuitry 1692, and antenna(s) 1699 incombination may form the wireless control device as discussed herein.

In one embodiment, an apparatus (e.g., hub) for providing a wirelessasymmetric network architecture includes a memory for storinginstructions, processing logic (e.g., one or more processing units,processing logic 1544, processing logic 1663, processing logic 1688,processing logic 1763, processing logic 1888) of the hub to executeinstructions to establish and control communications in a wirelessasymmetric network architecture, and radio frequency (RF) circuitry(e.g., RF circuitry 1550, RF circuitry 1670, RF circuitry 1692, RFcircuitry 1890) including multiple antennas (e.g., antenna(s) 1552,antenna(s) 1678, antenna(s) 1699, antennas 1311, 1312, and 1313, etc.)to transmit and receive communications in the wireless asymmetricnetwork architecture. The RF circuitry and multiple antennas to transmitcommunications to a plurality of sensor nodes (e.g., node 1, node 2)each having a wireless device with a transmitter and a receiver (ortransmitter and receiver functionality of a transceiver) to enablebi-directional communications with the RF circuitry of the apparatus inthe wireless asymmetric network architecture.

In one example, a first wireless node includes a wireless device withone or more processing units and RF circuitry for transmitting andreceiving communications in the wireless network architecture includinga first RF signal having a first packet. A second wireless node includesa wireless device with a transmitter and a receiver to enablebi-directional communications with the first wireless node in thewireless network architecture including a second RF signal with a secondpacket. The one or more processing units of the first wireless node areconfigured to execute instructions to determine a time of flightestimate for localization based on a time estimate of round trip time ofthe first and second packets and a time estimate of the time of flightthat is based on channel sense information of the first and secondwireless nodes.

In one example, the apparatus is powered by a mains electrical sourceand the plurality of sensor nodes are each powered by a battery sourceto form the wireless network architecture.

Various batteries could be used in the wireless sensor nodes, includinglithium-based chemistries such as Lithium Ion, Lithium Polymer, LithiumPhosphate, and other such chemistries as would be apparent to one ofordinary skill in the art. Additional chemistries that could be usedinclude Nickel metal hydride, standard alkaline battery chemistries,Silver Zinc and Zinc Air battery chemistries, standard Carbon Zincbattery chemistries, lead Acid battery chemistries, or any otherchemistry as would be obvious to one of ordinary skill in the art.

The present invention also relates to an apparatus for performing theoperations described herein. This apparatus may be specially constructedfor the required purposes, or it may comprise a general purpose computerselectively activated or reconfigured by a computer program stored inthe computer. Such a computer program may be stored in a computerreadable storage medium, such as, but not limited to, any type of diskincluding floppy disks, optical disks, CD-ROMs, and magnetic-opticaldisks, read-only memories (ROMs), random access memories (RAMs), EPROMs,EEPROMs, magnetic or optical cards, or any type of media suitable forstoring electronic instructions.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various general purposesystems may be used with programs in accordance with the teachingsherein, or it may prove convenient to construct a more specializedapparatus to perform the required method operations.

FIG. 15 illustrates a block diagram of a sensor node in accordance withone embodiment. The sensor node 1700 includes a power source 1710 (e.g.,energy source, battery source, primary cell, rechargeable cell, etc.)that provides power (e.g., DC power supply) to a controller circuit 1720via a connection 1774 (e.g., communication link, signal line, electricalconnection, etc.), provides power to RF circuitry 1770 via a connection1776 (e.g., communication link, signal line, electrical connection,etc.), and provides power to sensing circuitry 1740 via a connection1746 (e.g., communication link, signal line, electrical connection,etc.). The controller circuit 1720 includes memory 1761 or is coupled tomemory that stores instructions which are executed by processing logic1763 (e.g., one or more processing units) of the controller circuit 1720for controlling operations of the sensor node for forming and monitoringthe wireless asymmetrical network as discussed herein. The RF circuitry1770 (e.g., communication circuitry) may include a transceiver orseparate transmitter 1775 and receiver 1777 functionality for sendingand receiving bi-directional communications via antenna(s) 1778 with thehub(s) and optional wireless sensor nodes. The RF circuitry 1770communicates bi-directionally with the controller circuit 1720 via aconnection 1772 (e.g., electrical connection). The sensing circuitry1740 includes various types of sensing circuitry and sensor(s) includingimage sensor(s) and circuitry 1742, moisture sensor(s) and circuitry1743, temperature sensor(s) and circuitry, humidity sensor(s) andcircuitry, air quality sensor(s) and circuitry, light sensor(s) andcircuitry, motion sensor(s) and circuitry 1744, audio sensor(s) andcircuitry 1745, magnetic sensor(s) and circuitry 1746, and sensor(s) andcircuitry n, etc.

The wireless localization techniques disclosed herein may be combinedwith other sensed information to improve localization accuracy of theoverall network. For example, in wireless sensors in which one or moreof the nodes contain cameras, captured images can be used with imageprocessing and machine learning techniques to determine whether thesensor nodes that are being monitored are looking at the same scene andare therefore likely in the same room. Similar benefits can be achievedby using periodic illumination and photodetectors. By strobing theillumination and detecting using the photodetectors, the presence of anoptical path can be detected, likely indicating the absence of opaquewalls between the strobe and the detector. In other embodiments,magnetic sensors can be integrated into the sensor nodes and used as acompass to detect the orientation of the sensor node that is beingmonitored. This information can then be used along with localizationinformation to determine whether the sensor is on the wall, floor,ceiling, or other location.

In one example, each sensor node may include an image sensor and eachperimeter wall of a house includes one or more sensor nodes. A hubanalyzes sensor data including image data and optionally orientationdata along with localization information to determine absolute locationsfor each sensor node. The hub can then build a three dimensional imageof each room of a building for a user. A floor plan can be generatedwith locations for walls, windows, doors, etc. Image sensors may captureimages indicating a change in reflections that can indicate homeintegrity issues (e.g., water, leaking roof, etc.).

FIG. 16 illustrates a block diagram of a system 1800 having a hub inaccordance with one embodiment. The system 1800 includes or isintegrated with a hub 1882 or central hub of a wireless asymmetricnetwork architecture. The system 1800 (e.g., computing device, smart TV,smart appliance, communication system, etc.) may communicate with anytype of wireless device (e.g., cellular phone, wireless phone, tablet,computing device, smart TV, smart appliance, etc.) for sending andreceiving wireless communications. The system 1800 includes a processingsystem 1810 that includes a controller 1820 and processing units 1814.The processing system 1810 communicates with the hub 1882, anInput/Output (I/O) unit 1830, radio frequency (RF) circuitry 1870, audiocircuitry 1860, an optics device 1880 for capturing one or more imagesor video, an optional motion unit 1844 (e.g., an accelerometer,gyroscope, etc.) for determining motion data (e.g., in three dimensions)for the system 1800, a power management system 1840, andmachine-accessible non-transitory medium 1850 via one or morebi-directional communication links or signal lines 1898, 1818, 1815,1816, 1817, 1813, 1819, 1811, respectively.

The hub 1882 includes a power supply 1891 that provides power (e.g., DCpower supply) to a controller circuit 1884 via a connection 1885 (e.g.,communication link, signal line, electrical connection, etc.) andprovides power to RF circuitry 1890 via a connection 1887 (e.g.,communication link, signal line, electrical connection, etc.). Thecontroller circuit 1884 includes memory 1886 or is coupled to memorythat stores instructions which are executed by processing logic 1888(e.g., one or more processing units) of the controller circuit 1884 forcontrolling operations of the hub for forming and monitoring thewireless asymmetrical network as discussed herein. The RF circuitry 1890may include a transceiver or separate transmitter (TX) 1892 and receiver(RX) 1894 functionality for sending and receiving bi-directionalcommunications via antenna(s) 1896 with the wireless sensor nodes orother hubs. The RF circuitry 1890 communicates bi-directionally with thecontroller circuit 1884 via a connection 1889 (e.g., communication link,signal line, electrical connection, etc.). The hub 1882 can be awireless control device 1884 or the controller circuit 1884, RFcircuitry 1890, and antenna(s) 1896 in combination may form the wirelesscontrol device as discussed herein.

RF circuitry 1870 and antenna(s) 1871 of the system or RF circuitry 1890and antenna(s) 1896 of the hub 1882 are used to send and receiveinformation over a wireless link or network to one or more otherwireless devices of the hubs or sensors nodes discussed herein. Audiocircuitry 1860 is coupled to audio speaker 1862 and microphone 1064 andincludes known circuitry for processing voice signals. One or moreprocessing units 1814 communicate with one or more machine-accessiblenon-transitory mediums 1850 (e.g., computer-readable medium) viacontroller 1820. Medium 1850 can be any device or medium (e.g., storagedevice, storage medium) that can store code and/or data for use by oneor more processing units 1814. Medium 1850 can include a memoryhierarchy, including but not limited to cache, main memory and secondarymemory.

The medium 1850 or memory 1886 stores one or more sets of instructions(or software) embodying any one or more of the methodologies orfunctions described herein. The software may include an operating system1852, network services software 1856 for establishing, monitoring, andcontrolling wireless asymmetric network architectures, communicationsmodule 1854, and applications 1858 (e.g., home or building securityapplications, home or building integrity applications, developerapplications, etc.). The software may also reside, completely or atleast partially, within the medium 1850, memory 1886, processing logic1888, or within the processing units 1814 during execution thereof bythe device 1800. The components shown in FIG. 18 may be implemented inhardware, software, firmware or any combination thereof, including oneor more signal processing and/or application specific integratedcircuits.

Communication module 1854 enables communication with other devices. TheI/O unit 1830 communicates with different types of input/output (I/O)devices 1834 (e.g., a display, a liquid crystal display (LCD), a plasmadisplay, a cathode ray tube (CRT), touch display device, or touch screenfor receiving user input and displaying output, an optional alphanumericinput device).

In one example, an asynchronous system for localization of nodes in awireless network architecture comprises a first wireless node having awireless device with one or more processing units and RF circuitry fortransmitting and receiving communications in the wireless networkarchitecture including a first RF signal having a first packet and asecond wireless node having a wireless device with a transmitter and areceiver to enable bi-directional communications with the first wirelessnode in the wireless network architecture including a second RF signalwith a second packet. The one or more processing units of the firstwireless node are configured to execute instructions to determine acoarse time of flight estimate of the first and second packets and afine time estimate of the time of flight using channel sense informationof the first and second wireless nodes.

In another example, the first wireless node has a first reference clocksignal and the second wireless node has a second reference clock signal.

In another example, the coarse time of flight estimate of the first andsecond packets is based on a first time when the first wireless nodesends the first packet, a second time when the second wireless nodereceives the first packet, a third time when the second wireless nodesends the second packet, and a fourth time when the first wireless nodereceives the second packet.

In another example, the channel information of the first wireless nodecomprises a first measurement of a channel response of the second packetand the channel information of the second wireless device comprises asecond measurement of a channel response of the first packet.

In another example, a combined forward channel response for coarse andfine channel estimates comprises applying a clock phase offset as anaddition of a linear phase to the second measurement. In anotherexample, a combined reverse channel response for coarse and fine channelestimates comprises applying a clock phase offset as a subtraction of alinear phase to the first measurement.

In another example, at least one of Matrix Pencil and MUSIC algorithmsare used to estimate minimum delay of multiple paths from the firstmeasurement of the channel response of the second packet and the secondmeasurement of the channel response of the first packet. In anotherexample, a distance between the first and second wireless nodes is usedto determine relative or absolute locations of the first and secondwireless nodes based on anchor node based triangulation or anchor nodeless triangulation.

In another example, the distance between the first and second wirelessnodes is used to determined relative or absolute locations of the firstand second wireless nodes based on anchor node based triangulation oranchor node less triangulation.

In another example, the distance between the first and second wirelessnodes is used to determine localization information which is used todefine constellation membership in a wireless sensor network having aplurality of wireless sensor nodes. In another example, a remote devicewith one or more processing units that are configured to executeinstructions to at least partially determine a coarse time of flightestimate of the first and second packets and a fine time estimate of thetime of flight using channel information of the first and secondwireless nodes.

In one embodiment, a synchronous system for localization of nodes in awireless network architecture comprises a first wireless node having awireless device with one or more processing units and RF circuitry fortransmitting and receiving communications in the wireless networkarchitecture including a first RF signal having a first packet. A secondwireless node includes a wireless device with one or more processingunits and RF circuitry to enable bi-directional communications with thefirst wireless node in the wireless network architecture including asecond RF signal having a second packet. The one or more processingunits of the first wireless node are configured to execute instructionsto determine a coarse time of flight estimate of the first and secondpackets and a fine time estimate of the time of flight using channelinformation of the first and second wireless nodes. The first and secondwireless nodes have the same reference clock signal.

In one example, the coarse time of flight estimate of the first andsecond packets is based on a first time when the first wireless nodesends the first packet, a second time when the second wireless nodereceives the first packet, a third time when the second wireless nodesends the second packet, and a fourth time when the first wireless nodereceives the second packet.

The coarse time of flight estimate has a resolution that is limited by atime resolution of a sampling clock, which will have a frequency off_(s) for controlling circuitry that detects a timing of a transmissionor a reception.

In another example, the channel information of the first wireless nodecomprises a first measurement of a channel response of the second packetand channel information of the second wireless device comprises a secondmeasurement of a channel response of the first packet.

In another example, a combined channel response for coarse and finechannel estimates comprises applying a coarse delay as an addition of alinear phase shift to the channel response prior to estimating minimumdelay of multiple paths between the first and second wireless nodes.

In another example, at least one of Matrix Pencil and MUSIC algorithmsare used to estimate minimum delay of multiple paths between the firstand second wireless nodes.

In another example, the channel response without the coarse delay isaveraged across multiple packets or multiple wireless transmissions toimprove signal to noise ratio and thus improve accuracy of pathestimation.

In another example, the combined channel response with the coarse delayis averaged across multiple packets or multiple wireless transmissionsto improve signal to noise ratio and thus improve accuracy of pathestimation.

In one embodiment, an apparatus, comprises a memory for storinginstructions, one or more processing units to execute instructions forcontrolling a plurality of sensor nodes in a wireless networkarchitecture and determining locations of the plurality of sensor nodes,and radio frequency (RF) circuitry to transmit communications to andreceive communications from the plurality of sensor nodes each having awireless device with a transmitter and a receiver to enablebi-directional communications with the RF circuitry of the apparatus inthe wireless network architecture. The one or more processing units ofthe apparatus are configured to execute instructions to transmit a firstRF signal having a first packet to a sensor node, to receive a second RFsignal with a second packet from the sensor node, and to determine acoarse time of flight estimate of the first and second packets and afine time estimate of the time of flight using channel information ofthe first and second wireless nodes.

In one example, the apparatus has a first reference clock signal and thesensor node has a second reference clock signal.

In another example, the coarse time of flight estimate of the first andsecond packets is based on a first time when the apparatus sends thefirst packet, a second time when the sensor node receives the firstpacket, a third time when the sensor node sends the second packet, and afourth time when the apparatus receives the second packet.

In another example, channel information of the apparatus comprises afirst measurement of a channel response of the second packet and channelinformation of the sensor node comprises a second measurement of achannel response of the first packet.

In one example, a combined channel response comprises multiplying thefirst measurement and the second measurement to cancel a phasedifference between the first and second reference clock signals. Thecombined channel response is averaged across multiple packets ormultiple wireless transmissions to improve signal to noise ratio andthus improve accuracy of path estimation.

In another example, a divided channel response comprises dividing thefirst measurement by the second measurement to estimate a phasedifference between the first and second reference clock signals. Thedivided channel response is averaged across multiple packets or multiplewireless transmissions to improve signal to noise ratio and thus improveaccuracy of path estimation.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention.The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

What is claimed is:
 1. An asynchronous system for localization of nodesin a wireless network architecture, comprising: a first wireless nodehaving a wireless device with processing circuitry and RF circuitry fortransmitting and receiving communications in the wireless networkarchitecture including a first RF signal having a first packet; and asecond wireless node having a wireless device with a transmitter and areceiver to enable bi-directional communications with the first wirelessnode in the wireless network architecture including a second RF signalwith a second packet, wherein the processing circuitry of the firstwireless node is configured to execute instructions to determine acoarse time of flight estimate of the first and second packets based ona coarse sample period of a sampling clock and a fine time estimate ofthe time of flight using channel information of the first and secondwireless nodes, wherein a fine time estimate value of the fine timeestimate is less than one coarse sample period.
 2. The asynchronoussystem of claim 1, wherein the first wireless node has a first referenceclock signal and the second wireless node has a second reference clocksignal.
 3. The asynchronous system of claim 2, wherein the coarse timeof flight estimate of the first and second packets is based on a firsttime when the first wireless node sends the first packet, a second timewhen the second wireless node receives the first packet, a third timewhen the second wireless node sends the second packet, and a fourth timewhen the first wireless node receives the second packet.
 4. Theasynchronous system of claim 3, wherein the coarse time of flightestimate has a resolution that is limited by a time resolution of asampling clock, which will have a frequency of fs for controllingcircuitry that detects a timing of a transmission or a reception.
 5. Theasynchronous system of claim 3, wherein the channel information of thefirst wireless node comprises a first measurement of a channel responseof the second packet and the channel information of the second wirelessdevice comprises a second measurement of a channel response of the firstpacket.
 6. The asynchronous system of claim 5, wherein a combinedforward channel response for coarse and fine channel estimates comprisesapplying a clock phase offset as an addition of a linear phase to thesecond measurement.
 7. The asynchronous system of claim 6, wherein acombined reverse channel response for coarse and fine channel estimatescomprises applying a clock phase offset as a subtraction of a linearphase to the first measurement.
 8. The asynchronous system of claim 7,wherein at least one of Matrix Pencil and MUSIC algorithms are used toestimate minimum delay of multiple paths from the combined forwardchannel response and the combined reverse channel response.
 9. Theasynchronous system of claim 8, wherein a distance between the first andsecond wireless nodes is used to determined relative or absolutelocations of the first and second wireless nodes based on anchor nodebased triangulation or anchor-less node triangulation.
 10. Theasynchronous system of claim 8, wherein a distance between the first andsecond wireless nodes is used to determine localization informationwhich is used to define constellation membership in a wireless sensornetwork having a plurality of wireless sensor nodes.
 11. Theasynchronous system of claim 1, further comprising: a remote device withprocessing circuitry that is configured to execute instructions to atleast partially determine the coarse time of flight estimate of thefirst and second packets and the fine time estimate of the time offlight using channel information of the first and second wireless nodes.12. A synchronous system for localization of nodes in a wireless networkarchitecture, comprising: a first wireless node having a wireless devicewith processing circuitry and RF circuitry for transmitting andreceiving communications in the wireless network architecture includinga first RF signal having a first packet; and a second wireless nodehaving a wireless device with processing circuitry and RF circuitry toenable bi-directional communications with the first wireless node in thewireless network architecture including a second RF signal having asecond packet, wherein the processing circuitry of the first wirelessnode is configured to execute instructions to determine a coarse time offlight estimate of the first and second packets based on a coarse sampleperiod of a sampling clock and a fine time estimate of the time offlight using channel information of the first and second wireless nodes,wherein the first and second wireless nodes have the same referenceclock signal, wherein a fine time estimate value of the fine timeestimate is less than one coarse sample period.
 13. The synchronoussystem of claim 12, wherein the coarse time of flight estimate of thefirst and second packets is based on a first time when the firstwireless node sends the first packet, a second time when the secondwireless node receives the first packet, a third time when the secondwireless node sends the second packet, and a fourth time when the firstwireless node receives the second packet.
 14. The synchronous system ofclaim 13, wherein the coarse time of flight estimate has a resolutionthat is limited by a time resolution of a sampling clock, which willhave a frequency of fs for controlling circuitry that detects a timingof a transmission or a reception.
 15. The synchronous system of claim14, wherein channel information of the first wireless node comprises afirst measurement of a channel response of the second packet and channelinformation of the second wireless device comprises a second measurementof a channel response of the first packet.
 16. The synchronous system ofclaim 15, wherein a combined channel response for coarse and finechannel estimates comprises applying a coarse delay as an addition of alinear phase shift to the channel response prior to estimating minimumdelay of multiple paths between the first and second wireless nodes. 17.The synchronous system of claim 16, wherein at least one of MatrixPencil and MUSIC algorithms are used to estimate minimum delay ofmultiple paths between the first and second wireless nodes.
 18. Thesynchronous system of claim 16, wherein the channel response without thecoarse delay is averaged across multiple packets or multiple wirelesstransmissions to improve signal to noise ratio and thus improve accuracyof path estimation.
 19. The synchronous system of claim 16, wherein thecombined channel response with the coarse delay is averaged acrossmultiple packets or multiple wireless transmissions to improve signal tonoise ratio and thus improve accuracy of path estimation.
 20. Anapparatus, comprising: a memory for storing instructions; processingcircuitry to execute instructions for controlling a plurality of sensornodes in a wireless network architecture and determining locations ofthe plurality of sensor nodes; and radio frequency (RF) circuitry totransmit communications to and receive communications from the pluralityof sensor nodes each having a wireless device with a transmitter and areceiver to enable bi-directional communications with the RF circuitryof the apparatus in the wireless network architecture, wherein theprocessing circuitry of the apparatus is configured to executeinstructions to transmit a first RF signal having a first packet to asensor node, to receive a second RF signal with a second packet from thesensor node, and to determine a coarse time of flight estimate of thefirst and second packets based on a coarse sample period of a samplingclock and a fine time estimate of the time of flight using channelinformation of the sensor node and the apparatus, wherein a fine timeestimate value of the fine time estimate is less than one coarse sampleperiod.
 21. The apparatus of claim 20, wherein the apparatus has a firstreference clock signal and the sensor node has a second reference clocksignal.
 22. The apparatus of claim 21, wherein the coarse time of flightestimate of the first and second packets is based on a first time whenthe apparatus sends the first packet, a second time when the sensor nodereceives the first packet, a third time when the sensor node sends thesecond packet, and a fourth time when the apparatus receives the secondpacket.
 23. The apparatus of claim 22, wherein channel information ofthe apparatus comprises a first measurement of a channel response of thesecond packet and channel information of the sensor node comprises asecond measurement of a channel response of the first packet.
 24. Theapparatus of claim 23, wherein a combined channel response comprisesmultiplying the first measurement and the second measurement to cancel aphase difference between the first and second reference clock signals,wherein the combined channel response is averaged across multiplepackets or multiple wireless transmissions to improve signal to noiseratio and thus improve accuracy of path estimation.
 25. The apparatus ofclaim 23, wherein a divided channel response comprises dividing thefirst measurement by the second measurement to estimate a phasedifference between the first and second reference clock signals, whereinthe divided channel response is averaged across multiple packets ormultiple wireless transmissions to improve signal to noise ratio andthus improve accuracy of path estimation.